Heart sounds are associated with mechanical vibrations from activity of a subject's heart and the flow of blood through the heart. Heart sounds recur with each cardiac cycle and are separated and classified according to the activity associated with the vibration. For example, the first heart sound (S1) is associated with the vibrational sound made by the heart during tensing of the mitral valve. The second heart sound (S2) marks the beginning of diastole. The third heart sound (S3) and fourth heart sound (S4) are related to filling of the left ventricle during diastole.
Implantable medical devices (IMDs) are devices designed to be implanted into a subject. Some examples of these devices include cardiac function management (CFM) devices such as implantable pacemakers, implantable cardioverter defibrillators (ICDs), cardiac resynchronization devices, neural stimulation devices, and devices that include a combination of such capabilities, among other devices. The devices are typically used to treat subjects using electrical therapy or to aid a physician or caregiver in subject diagnosis through internal monitoring of a subject's condition. Some devices include electrodes in communication with sense amplifiers to monitor electrical activity within a subject, and some include sensors, such as impedance sensors or pulmonary artery pressure sensors, to monitor other internal subject parameters, such as thoracic impedance or pulmonary artery pressure. Other examples of implantable medical devices include implantable diagnostic devices, implantable insulin pumps, devices implanted to administer drugs to a subject, or implantable devices with neural stimulation capability.
Patangay et al., in U.S. Patent Application Publication No. 2008/0103399, now issued as U.S. Pat. No. 8,364,263, entitled SYSTEM AND METHOD FOR SYSTOLIC INTERVAL ANALYSIS, refers to calculating a time interval using a transformed cardiac impedance signal and a heart sound signal. Patangay et al., in U.S. Patent Application Publication No. 2011/0034812, abandoned, entitled PULMONARY ARTERY PRESSURE BASED SYSTOLIC TIMING INTERVALS AS A MEASURE OF RIGHT VENTRICULAR SYSTOLIC PERFORMANCE, refers to identifying timing intervals using a pulmonary artery pressure (PAP) signal.